A so-called trochoid pump is generally used as an oil pump for lubricating an internal combustion engine for an automobile, for example.
An oil pump disclosed in Japanese Patent No. JP10-77973A, which is hereby incorporated by reference in its entirety herein, includes an inner rotor, which is accommodated in a rotor receiving portion formed at an inner side of a housing and to which rotations of a drive shaft is transmitted via a pair of extending portions that are formed at a bearing bore where the drive shaft is fitted and that extend in a radially inward direction so as to face each other. In addition, the oil pump includes an outer rotor accommodated in the rotor receiving portion in such a way that the outer rotor is rotatable in relation to the rotation of the inner rotor about a rotational center eccentric relative to a rotational center of the inner rotor by a predetermined amount. The outer rotor includes multiple inner teeth engaging with multiple outer teeth formed at the inner rotor to thereby define multiple pump chambers therebetween. Then, protruding portions are formed at one side of the respective extending portions of the inner rotor so as to extend in an axial direction and to be inserted into an inner bore of the housing. A length or diameter between outer peripheral surfaces of the respective protruding portions is defined in such a way that the protruding portions are prevented from making slidably contact with the inner bore of the housing at a time of rotations of the inner rotor. As a result, the assembly performance of the inner rotor relative to the housing can be ensured without an increase of sliding loss at a time of rotations of the inner rotor.
According to the oil pump disclosed in JP10-77973A, a positioning of the inner rotor is conducted by defining a clearance as small as possible between an inner surface of the bearing bore and an outer surface of the drive shaft. Then, due to a clearance formed between an inner surface of the rotor receiving portion where the outer rotor is accommodated, and an outer surface of the outer rotor, a possible run-out of the drive shaft that is caused by a vibration of an engine, and the like, can be absorbed.
For example, the rotational center of the outer rotor is defined to be eccentric relative to the rotational center of the inner rotor on a plump line thereof by a predetermined amount. Then, the outer rotor receives a load such as driving force transmitted from the inner rotor and hydraulic pressure of operational oil. In such a structure, in the cases where the driving force is large, the outer rotor is biased to one side in a horizontal direction, for example. In addition, in the cases where the hydraulic pressure is high, the outer rotor is biased to the other side in the horizontal direction, for example. Accordingly, when the load biasing the outer rotor in directions opposite from each other occurs simultaneously and frequently, a run-out of the outer rotor occurs frequently within the clearance defined between the inner surface of the rotor receiving portion and the outer surface of the outer rotor. At this time, a hitting sound such as clanking is generated between the inner teeth of the outer rotor and the outer teeth of the inner rotor.
Thus, a need exists for an oil pump which is not susceptible to the drawbacks mentioned above.